The present invention relates to a portable personal computer of the laptop type or the notebook type, and more particularly to a computer system having the function of controlling the operating speed of the CPU by means of a stop clock signal for reducing power consumption and to a stop clock signal control method for use in the system.
In recent years, various portable personal computers of the laptop type or the notebook type have been developed. With this type of computer, as shown in FIG. 1A, the system controller 32 supplies to the CPU 31 a signal, called a stop clock signal (signal STPCLK#), for controlling the operating speed of the CPU 31. Depending on the logical state of the signal STPCLK#, the CPU 31 is temporarily stopped from executing an instruction, thereby reducing the power consumption.
To realize a reduction in the power consumption by using the signal STPCLK#, the computer system of FIG. 1A is provided with a specific mode called interval stop clock mode.
The user can choose the mode on the setup screen. When the mode is chosen, the controller 32 outputs to the CPU 31 the signal STPCLK# that alternates between an active period (in low level) and an inactive period (in high level) (e.g., at intervals of 32 .mu.s). According to the alternation of signal STPCLK# between the active period and the inactive period, the CPU 31 alternates between a first state in which the CPU is temporarily stopped from executing an instruction and a second state in which the CPU is allowed to execute the instruction.
As described above, in the interval stop clock mode, the CPU 31 alternates between the temporary stop period of instruction execution and the instruction executable period, thereby assuring the reduction of power consumption. Because the CPU stops executing an instruction intermittently, but not completely, it can perform processing, although the speed is low and therefore the performance falls off. Therefore, the interval stop clock mode is a useful function when the user uses the computer system operating on a battery.
With a conventional computer system, however, once the interval stop clock mode has been set, the CPU is temporarily stopped intermittently, regardless of the state of the CPU. As a result, when a load is applied to the CPU, the CPU has the disadvantage of degrading the performance. The degradation of the CPU's performance is a problem, especially when a high-speed hardware device (I/O device), such as a hard disk drive (HDD), has triggered an interrupt to the CPU.